EarthArXiv Coversheet 2021/01/12 Dynamic recrystallization by subgrain rotation in olivine revealed by high-spatial resolution electron backscatter diffraction Marco A. Lopez-Sanchez* Géosciences Montpellier – CNRS & Université de Montpellier, France Andrea Tommasi, Géosciences Montpellier – CNRS & Université de Montpellier, France Walid Ben Ismail Rock Deformation Laboratory, Dept. Earth Sciences, University of Manchester, UK. Now at K&M Technology Group (Houston, US) Fabrice Barou Géosciences Montpellier – CNRS & Université de Montpellier, France *corresponding author: [email protected] This manuscript has been submitted for publication in TECTONOPHYSICS. This is therefore a non-peer reviewed preprint submitted to EarthArXiv and thus may be periodically revised. If accepted, the final version will be available via the ‘Peer-review Publication DOI’ link on the right hand side of this webpage. Please feel free to contact any of the authors; we welcome feedback. Highlights We document how subgrain rotation (SGR) recrystallization develops in olivine SGR preferentially occurs in areas subjected to local stress concentrations The formation of subgrain cells requires the activation of hard slip systems The misorientation axis change at the subgrain to grain boundary (GB) transition We propose that this change marks the creation of new defects at the new GBs 1 Dynamic recrystallization by subgrain 2 rotation in olivine revealed by high-spatial 3 resolution electron backscatter diffraction 4 Marco A. Lopez-Sanchez1*, Andrea Tommasi1, Walid Ben Ismail2,3 and 5 Fabrice Barou1 6 1Géosciences Montpellier – CNRS & Université de Montpellier, France 7 2Rock Deformation Laboratory, Dept. Earth Sciences, University of Manchester, UK 8 3Now at K&M Technology Group (Houston, US) 9 10 E-mail: [email protected] 11 Tel.: +33 467 143 064. 12 *Corresponding author 13 1 14 Abstract 15 We document how dynamic recrystallization by subgrain rotation (SGR) develops in 16 natural olivine-rich rocks deformed in extension to up to 50% bulk finite strain (1473 K, 17 confining pressure of 300 MPa, and stresses between 115-180 MPa) using high- 18 resolution electron backscatter diffraction (EBSD) mapping. SGR occurs preferentially 19 in highly deformed grains (well-oriented to deform by dislocation glide) subjected to 20 local stress concentrations due to interactions with hard grains (poorly-oriented olivine 21 crystals or pyroxenes). Subgrains (misorientation <15°) are delimited mainly by tilt 22 walls composed by combinations of dislocations of the [100](001), [001](100), 23 [100](010) and [001](010) systems, in order of decreasing frequency. The activation 24 and prevalence of these systems agree with a Schmid factor analysis using values for 25 high-T deformation in olivine. The development of closed 3D subgrain cells by SGR 26 recrystallization requires the contribution of at least three different slip systems, 27 implying the activation of hard slip systems and high (local) stresses. The transition 28 from subgrains to grain boundaries (misorientation ≥ 15°) is characterized by a sharp 29 change in the misorientation axes that accommodate the difference in orientation 30 between the two subgrains or grains. We propose that this change marks the creation 31 and incorporation of new defects (grain boundary dislocations with different Burger 32 vectors and possibly disclinations or disconnections) at the newly-formed grain 33 boundaries. This process might be favoured by stress concentrations at the grain 34 boundaries due to increasing misalignment between slip systems in parent and 35 recrystallized grains. Finally, we document the development of strong misorientations 36 due to accumulation of low angle grain boundaries within the parent grains and between 37 them and the recrystallized ones. This process leads to intense dispersion of the crystal 38 preferred orientation resulting from SGR alone, with no involvement of grain boundary 39 sliding. 40 Keywords: Olivine; Dynamic recrystallization; Subgrain rotation recrystallization; 41 Geometrically necessary dislocations; Electron backscatter diffraction; Misorientation 42 2 43 1. Introduction 44 When a large ductile (viscoplastic) strain occurs in the lithosphere, all major rock- 45 forming minerals undergo dynamic recrystallization (DRX). DRX modifies the 46 microstructure (grain size and shape, grain boundary arrangement) and the 47 crystallographic preferred orientation (CPO), and decreases the dislocation content of 48 the rock. By consequence, DRX decreases the strength and changes the anisotropy of 49 physical properties of rocks. These changes can potentially enable or disable strain 50 localization by varying the strength distribution within a rock volume. Understanding 51 how dynamic recrystallization evolves at the microscale is thus key for determining 52 how rocks respond to deformation up to lithospheric scales. This knowledge is essential 53 for mineral phases like olivine, which makes up most of the lithosphere and the upper 54 convective mantle. 55 DRX involves the creation and/or migration of high-angle grain boundaries (HAGB; 56 >15 in olivine) during plastic deformation to form new grains with a lower dislocation 57 content than the host material (Doherty et al., 1997; Urai et al., 1986). Two main DRX 58 mechanism exists, grain boundary migration (GBM) and subgrain rotation (SGR) 59 recrystallization (Guillope and Poirier, 1979; Sakai et al., 2014; Urai et al., 1986), which 60 are referred to as discontinuous and continuous dynamic recrystallization, respectively, 61 in materials science (Huang and Logé, 2016). Both mechanisms may operate alone or in 62 parallel with one dominating the other. Here we focus on subgrain rotation 63 recrystallization, a ubiquitous DRX mechanism in rock-forming minerals. SGR 64 generates new grains with increasing strain by progressive rotation of the so-called 65 subgrains cells due to incorporation of dislocations within the low angle boundaries 66 (LAB) that delimit the cells, with limited (or lack of) grain boundary migration (Drury 67 and Pennock, 2007; Ion et al., 1982; Poirier and Guillope, 1979; Poirier and Nicolas, 68 1975). 69 Despite the simple description of SGR recrystallization provided above, many of the 70 underlying processes remain poorly understood (Drury and Pennock, 2007; Huang and 71 Logé, 2016; Sakai et al., 2014). Missing knowledge concerns in particular the transition 72 from LAB to HAGB. For example, a study in calcite using electron backscatter 73 diffraction has observed randomization of the misorientation axes once LABs became 74 HAGBs (Bestmann and Prior, 2003). Is this a general evolution valid for other mineral 3 75 phases such as olivine? If so, is it solely due to the progressive accumulation of 76 geometrically necessary dislocations (GNDs) from the active slip systems or are other 77 processes involved? What are the processes behind this change? An additional question 78 in highly anisotropic materials of low symmetry such as olivine, quartz or ice, with few 79 slip systems and highly contrasted strength, is: How many slip systems have to be 80 activated to generate a closed subgrain structure that can evolve into a DRX grain? 81 These issues are central to the development of reliable numerical models to determine 82 how DRX affects the microstructure and the physical properties of rocks and other 83 polycrystalline materials during ductile (viscoplastic) deformation. 84 Our goal is to document: (i) How do subgrains form in olivine? (ii) How do they 85 evolve to form new grains? and (iii) How does this process modify the CPO of the 86 rock? For this, we analyse post-mortem high-resolution electron backscatter diffraction 87 (EBSD) maps of experimentally deformed olivine-rich rocks. This technique allows 88 combining high-resolution characterization of subgrain boundaries (determination of the 89 dislocation families that compose these boundaries) and of recrystallized grain 90 boundaries and statistical data on the CPO and misorientations across grain and 91 subgrain boundaries at the sample scale. 92 2. Methods 93 The studied samples are natural peridotites deformed in axial extension at a constant 94 displacement rate of ~10-5 s-1, 1473 K (±2), and confining pressure of 300 MPa (±1) 95 using a Paterson-type gas medium triaxial apparatus at the Rock Deformation 96 Laboratory, University of Manchester, UK. Two different starting materials were used 97 in these experiments: coarse-grained dunites from the Balmuccia massif in the Alps 98 (>96% Olivine Fo82-83), with either a weak (VS14) or strong (VS15) CPO, and a fine- 99 grained mylonitic harzburgite from Wuqba massif in the Oman ophiolite (75% Olivine 100 Fo91, 90OA87). The dunites display a well-equilibrated polygonal microstructure, with a 101 mean grain size ≥1 mm. The harzburgite has a mylonitic microstructure, with a bimodal 102 grain size distribution defined by elongated olivine porphyroclasts up to 1 mm long and 103 fine-grained equiaxial neoblasts 10-50 µm wide. 104 The present study focuses on DRX. Therefore, we selected for post-mortem analysis 105 of the microstructure the most deformed samples, with bulk strains (푙푓 − 푙0)/푙0 ranging 106 between 29.6 to 50.1 %. These samples exhibit non-uniform deformation, with a clear 4 107 necking zone, where strain and stress concentration led to DRX. We performed high- 108 resolution EBSD mapping of one to three strongly recrystallized domains in the necking